High intensity discharge lamps generally comprise an elongated arc tube containing an ionizable fill and having press seals at each end of the tube. Disposed within the arc tube are two main electrodes, one at each end. The electrodes are generally supported in the press seals and are usually connected to a thin molybdenum ribbon, disposed within the press seal. The thin ribbon prevents seal failures because of thermal expansion of the lead-in wire.
In order to facilitate starting of the gaseous discharge, an auxiliary starting electrode disposed in the arc tube adjacent one of the main electrodes has been used in the past. Such an electrode is used because an arc can be ignited between the starter electrode and its adjacent electrode at a much lower starting voltage than is required to ignite a discharge between the two main electrodes. Once the discharge is ignited, the ionizing gas provides primary electrons between the two main electrodes and if enough potential is available between the main electrodes a discharge will be formed therebetween. The starter electrode normally has a resistor in series with it to limit the current flowing through the auxiliary starting electrode after the discharge has started.
Unfortunately, the press sealed electrical feedthrough for the auxiliary starting electrode suffers a sodium electrolysis failure mechanism which may lead to premature seal failures. This mechanism is made worse at the elevated seal temperatures associated with the newer, low color temperature, high efficiency metal halide lamps. For these reasons, the starter electrode approach has generally been abandoned in favor of a high voltage starting pulse applied directly to the main electrodes of the arc tube. With this method, the seal failure problems associated with the starting electrode have been overcome, however, there is often delays from the time the high voltage is applied to the lamp electrodes to the time when the discharge occurs.
Metal halide lamps, on the whole, require higher voltages for reliable starting and operating than do high pressure mercury vapor lamps of corresponding size or rating. Conventional lead-lag ballasts for high pressure mercury vapor lamps do not deliver sufficient voltage for reliable starting As a result, circuits have been developed to further increase the voltage output delivered by a conventional lead-lag ballast during starting.
U.S. Pat. No. 4,808,888, which issued to Wyner et al on Feb. 28, 1989 and is assigned to the Assignee of the present Application, relates to a starting and operating circuit for high pressure discharge lamps. Although the circuit is effective with various high pressure discharge lamps, the circuit does not deliver sufficient voltage to reliably start metal halide lamps. Moreover, it has been found that the circuit produces a starting delay of from 2.0 to 3.0 minutes.
U.S. Pat. No. 4,721,888, which issued to Proud et al on Jan. 26, 1988, relates to an apparatus for starting and operating an arc discharge lamp which contains an ultraviolet enhancer. The ultraviolet enhancer illuminates the path between the electrodes of the arc tube so as to significantly decrease the starting time of the lamp. In the embodiment illustrated in FIGS. 1 and 2, the lamp requires an auxiliary starting electrode 3 and requires three resistors R11, R12, R14 and a diode. FIGS. 3 and 4 of the Proud et al patent illustrate embodiments of an arc discharge lamp without an auxiliary starting electrode. However, these latter embodiments include a spiral line generator 112 which may be relatively bulky.
U.S. Pat. Nos. 4,812,714 and 4,818,915, which issued respectively to Keeffe et al on Mar. 14, 1989 and Zaslavsky et al on Apr. 4, 1989 and are assigned to the same Assignee as the present Application, relate to metal halide arc discharge lamps which contain a source of ultraviolet radiation proximate the arc tube. The lamps described in these two patents operate in conjunction with an externally mounted ignitor 67 (FIG. 4) which is relatively expensive.